The present invention relates to a transparent coated plastic film of high transmittance for visible light and high reflectance for infrared radiation comprising a carrier film of a transparent polymer and a reflection metal layer embedded between two dielectric layers, and to a plastic laminate prepared therefrom.
Transparent coated plastic films are used as transparent heat reflectors, which are to be understood as optical filters which reflect infrared or heat radiation and are transparent to visible light, i.e. radiation in the visible region of the electromagnetic spectrum.
Such transparent heat filters should transmit the greatest possible proportion of visible light and reflect the greatest possible proportion of the incident heat radiation. These filters may be employed to prevent heating of the interior of a room by solar radiation.
In recent years, the automotive industry endeavoured to reduce the fuel consumption of motor vehicles. This is accomplished primarily by a marked reduction in the drag coefficients of the vehicles since, as is known, by far the major part of the energy required for moving the vehicle is needed for overcoming the air resistance. In the course of this development, the glass areas of the motor vehicles keep increasing in size and the laminated glass panes are thus arranged at increasing angles. As a consequence, the known "greenhouse effect" in the vehicle interior manifests itself much more strongly, especially in the summer months. In motor vehicles which are left to stand for four hours at an external temperature of, for example, 35.degree. C. in the sun, temperatures of up to 75.degree. C. are measured in the vehicle interior. An overheated interior represents a serious physical and psychic stress for the occupants of the vehicle, especially the driver, and is thus a factor in reducing road safety.
One way of preventing an undue rise of the temperatures in the vehicle interior, even in summer, is to provide the glass panes of the vehicle with a coating, which has a high transparency in the visible spectral region and is strongly reflecting throughout the infrared region.
U.S. Pat. No. 4,548,691 discloses a process for the manufacture of glass panes of high transmittance behavior in the visible region and high reflection behavior towards heat radiation in which flat glass panes are directly coated by cathodic sputtering. A first layer of the group comprising In.sub.2 O.sub.3, SnO.sub.2 or mixed oxides thereof is produced on the flat glass panes. A second layer of silver, a third layer of the group comprising Al, Ti, Ta, Cr, Mn and Zr and a fourth layer of the group comprising In.sub.2 O.sub.3, SnO.sub.2 or mixed oxides thereof are applied to this first layer. Transparent heat reflectors can be produced in this way, but the process has disadvantages: it is very involved and expensive to coat individual glass panes. A further difficulty is that, to manufacture windshields for motor vehicles, the flat, coated glass panes must undergo an additional bending process, which adversely affects the quality of the coating. Furthermore, a coating, which manages without the layer which serves purely to protect the second layer of silver, is desirable.
U.S. Pat. No. 4,465,736 discloses a light-transmitting film comprising a transparent polymer carrier film and a light-transmitting metal layer of a metal from the group comprising Ag, Au, Cu and alloys thereof. A dielectric layer of titanium, bismuth, tungsten, indium, zirconium or silicon oxide is applied to one or both sides of the metal layer. In addition, a protective layer of Ti, Zr, Se or C can also be formed on one or both sides of the metal layer. The light-transmitting film is bonded on both sides to polyvinyl butyral to give a laminate which is, for example, inserted between two glass panes and integrated with the latter to form a laminated safety glass pane.
The carrier film of the laminate is in general composed of polyethylene terephalate. The two dielectric layers applied to both sides of the metal layer are precipitated on the metal layer, for example, by hydrolysis of tetrabutyl titanate in alcohol.
It is an object of the present invention to provide an improved transparent coated plastic film of high transmittance for visible light and high reflectance for IR radiation which does not require a protective layer for the silver layer. Another object of the present invention is to provide a heat reflecting, transparent coated plastic film which is an essential constituent of a laminate in a laminated safety glass, which can be processed with equal ease and by the same methods as a conventional polyvinyl butyral film.